This invention relates to pyrethrum plants and, more particularly, to novel varieties of pyrethrum plants characterized by high pyrethrin content combined with environmental stress tolerance.
The flowers of Chrysanthemum cinerariaefolium, commonly known as pyrethrum, yield a family of six related secondary metabolites, collectively called pyrethrin, which exhibits excellent insecticidal properties. Pyrethrin has become a commercially important insecticide due to the fact that it has minimal mammalian toxicity, it photodegrades rapidly leaving no toxic residue in the environment, and it is highly effective against target organisms. It is the insecticide of choice for use in food manufacturing and in stable, dairy barn and pet fly and other insect control.
Economical commercial production of pyrethrin from Chrysanthemum cinerariaefolium requires plants whose flowers have a relatively high endogenous pyrethrin content, i.e., at least about 1.5%, and preferably about 2.0% or higher, by weight of dried flowers. Heretofore, such high levels of pyrethrin content have been reproducibly achievable only with Chrysanthemum cinerariaefolium grown in those parts of the world enjoying a relatively temperate climate with little or no frost and no excessive heat. The reason for this is the inverse correlation that has generally been found to exist between the vigor of this plant and its pyrethrin content. Thus, pyrethrin content is normally highest in the least thrifty and least vigorous plants, while those plants which do survive for a time in harsh climates have little or nearly no pyrethrin content. Consequently, commercial pyrethrin production has for the most part been limited to areas of the world either near the equator and at elevations sufficiently high so as to avoid heat stress while not so high as to encounter frost, or where tropical heat encountered at low elevations is moderated by coastal climatic effects.
Based on the foregoing, it has come to be generally accepted by Chrysanthemum cinerariaefolium growers and breeders that any variety of this plant having a relatively high level of pyrethrin content, would necessarily lack the environmental stress tolerance sufficient for long-term survival under the climatic extremes of both frost and excessive heat.